There is such a hollow speed reducer disclosed in, for example, Patent Reference 1 (JP-A-7-108485). According to Patent Reference 1, there is disclosed a rotation joint of a robot or the like characterized in a rotation joint of a robot or the like for decelerating a rotation output of a drive motor and rotating a rotation portion around a rotation axis center relative to a fixed portion, constituted by an outer bearing apparatus including a hollow speed reducer having an output portion for decelerating a rotation drive force of the drive motor to a rotation output around a predetermined axis center to transmit to the rotation portion and including a hollow cylindrical hole at a center portion, an outer bearing apparatus to which a stationary ring is locked by the fixing portion and in which a rotation ring is locked by an outer periphery of an output portion of the hollow speed reducer, and cylinder wall forming means provided at the hollow cylindrical hole of the hollow speed reducer and ensuring a cylinder region communicating utility regions on both sides in an axial direction of the hollow speed reducer to each other.
According to the speed reducer disclosed in Patent Reference 1, there is a problem that the motor is large-sized, and therefore, a turning structure is large-sized, particularly, a length in an axial direction is large.
As a countermeasure thereagainst, when a large output should be outputted by using a high speed/small sized motor and increase a speed reduction ratio of the speed reducer, in order to increase the speed reduction ratio, other speed reducing mechanism needs to be attached separately. Therefore, there is still other problem that the turning structure cannot be downsized.    Patent Reference 1: JP-A-7-108485    Patent Reference 2: JP-UM-A-4-115592    Patent Reference 3: JP-B-8-22516